5-DOF Hybrid Robot Research Articles

Dimensional synthesis of a 5-DOF hybrid robot

Combining the characteristics of parallel and series robots, hybrid robot has a good application prospect of machining. The dimensional synthesis, which is highly related to the performance, is studied based on a 5-DOF hybrid robot in this paper. The construction methods of optimization objectives which better reflect the motion range, stiffness and the kinematics performance are proposed. The bounding box theory is introduced to construct indexes of the actual motion ranges while considering the motion interference caused by mechanisms. And the maximum and minimum stiffness indexes of parallel structure based on the modified stiffness model and the comprehensive kinematics performance index based on condition number are also designed. Four constraints including installation, joint motion limitation, joint load limitation and validity of data define the feasible region. It is found from the optimization results that motion range changes more greatly than other objectives, which makes the solution with better motion ranges a higher priority. The correlation between dimensional parameters and optimization objectives is analyzed to guide the improvement of optimization model and provide references for dimensional synthesis of other hybrid robots.

Forward and inverse kinematics of a 5-DOF hybrid robot for composite material machining

Abstract Hybrid robots consist of both serial and parallel mechanisms, which have advantages in stiffness and workspace compared with serial/parallel robots when machining composite material. However, the forward and inverse kinematics of hybrid robots generally do not have analytic solutions. This paper deals with the analytic forward and inverse kinematics solutions of a 5-degree-of-freedom (DOF) hybrid robot which consists with a 3-DOF 2UPU/SP parallel mechanism (PM) and a 2-DOF rotating head. In the forward kinematic problem, a method is proposed to transfer the high order kinematic equation to a 4th-order polynomial based on the Sylvester's dialytic elimination, and the analytic solutions can be further obtained by Ferrari's method. In the inverse problem, the redundant Euler angles expressed by four rotations are firstly proposed for decoupling different motions, then, the closed-form solution of inverse kinematics can be found. Finally, a simulation trajectory is given, and the result shows that the accuracy of the solutions’ calculation reaches femtometer grade and the efficiency reaches microsecond grade; furthermore, an experiment is performed on the prototype to validate the effectiveness of the proposed forward and inverse kinematics.

An iterative tuning approach for feedforward control of parallel manipulators by considering joint couplings

Feedforward control is an effective way to improve the joint tracking accuracy of robotic systems. This paper presents an iterative approach for feedforward controller parameter tuning of parallel manipulators that considers joint couplings (cross-talk). Based upon a compound control strategy, increments of the feedforward tuning parameters are iteratively updated by minimizing the sum of squares of joint tracking errors at each step. A plant-free identification Jacobian is formulated using the measured data associated with a number of sequential statuses within each iteration cycle. Experiments on the 3-DOF parallel mechanism within a 5-DOF hybrid robot verify parameter convergence and the extrapolation capability of the proposed approach. Compared to otherwise similar feedforward control not considering joint couplings, the root mean square of joint tracking errors was reduced by up to 22% when the mechanism moved at high speed along a path in the neighborhood of the reference configuration.

An approach for elastodynamic modeling of hybrid robots based on substructure synthesis technique

Parallel kinematic machines exhibit strong pose-dependent static and dynamic behaviors, which makes accurate and rapid compliance analysis over the entire workspace an important issue in the design optimization. This paper presents a general approach for elastodynamic modeling of parallel kinematic machines using substructure synthesis technique. Firstly, the whole system is decomposed into two groups of substructures, i.e. component substructures and joint substructures. Then, the degrees of freedom of component substructures are reduced using modal reduction technique, and joint substructures are modeled by virtual springs with contact stiffness between adjacent component substructures. Finally, two threads are merged at junction surfaces to offer the equations of motion of the system as a whole, allowing the static and dynamic performances to be rapidly predicted with sufficient accuracy. The dynamic model of a 5-DOF hybrid robot is developed using the proposed method and the computational results show that rigidity and lower mode natural frequencies over the workspace match well with those obtained by the full finite element analysis.

Stiffness modeling and analysis of a novel 5-DOF hybrid robot

This paper deals with the stiffness modeling and analysis of a novel 5-DOF hybrid robot named TriMule which is composed of a 3-DOF positioning parallel mechanism plus a 2-DOF wrist. The robot is especially designed as a compact yet rigid module suitable for large part on-site machining. Mainly drawing on screw theory, a semi-analytic stiffness model of the robot is formulated by taking into account the component compliances associated with the elements of both the parallel mechanism and the wrist, resulting in the Cartesian stiffness matrix that can explicitly be expressed in terms of the compliance matrices down to the joint and link level. The stiffness distributions of the tool head over a prescribed task workspace are predicted and the contributions of joint/link compliances are evaluated using a set of global indices.

A New Methodology for Solving Trajectory Planning and Dynamic Load-Carrying Capacity of a Robot Manipulator

A new methodology using a direct method for obtaining the best found trajectory planning and maximum dynamic load-carrying capacity (DLCC) is presented for a 5-degree of freedom (DOF) hybrid robot manipulator. A nonlinear constrained multiobjective optimization problem is formulated with four objective functions, namely, travel time, total energy involved in the motion, joint jerks, and joint acceleration. The vector of decision variables is defined by the sequence of the time-interval lengths associated with each two consecutive via-points on the desired trajectory of the 5-DOF robot generalized coordinates. Then this vector of decision variables is computed in order to minimize the cost function (which is the weighted sum of these four objective functions) subject to constraints on joint positions, velocities, acceleration, jerks, forces/torques, and payload mass. Two separate approaches are proposed to deal with the trajectory planning problem and the maximum DLCC calculation for the 5-DOF robot manipulator using an evolutionary optimization technique. The adopted evolutionary algorithm is the elitist nondominated sorting genetic algorithm (NSGA-II). A numerical application is performed for obtaining best found solutions of trajectory planning and maximum DLCC calculation for the 5-DOF hybrid robot manipulator.

A semi-analytical approach for stiffness modeling of PKM by considering compliance of machine frame with complex geometry

Stiffness modeling is one of the most significant issues in the design of parallel kinematic machine (PKM). This paper presents a semi-analytical approach that enables the stiffness of PKM with complex machine frame geometry to be estimated effectively. This approach can be implemented by three steps: (i) decomposition of the entire system into two sub-systems associated with the parallel mechanism and the machine frame respectively; (ii) stiffness modeling of each sub-system using the analytical approach and the finite element analysis; and (iii) generation of the stiffness model of the entire system by means of linear superposition. In the modeling process of each sub-system, the virtual work principle and overall deflection Jacobian are employed with special attention to the bending rigidity of the constrained passive limb and the interface stiffness of the machine frame. The stiffness distribution of a 5-DOF hybrid robot named TriVariant-B is investigated as an example to illustrate the effectiveness of this approach. The contributions of component rigidities to that of the system are evaluated using global indices. It shows that the results achieved by this approach have a good match to those obtained through finite element analysis and experiments.

Kinematic design of 5-DOF hybrid robot with large workspace/limb-stroke ratio

This paper deals with the conceptual and kinematic designs of a 5-degree of freedom (DOF) reconfigurable hybrid robot. The robot is composed of a 2-DOF parallel spherical mechanism that is serially connected with a 3-DOF open loop kinematic chain via a prismatic joint. Somewhat similar to the well-known Tricept robot, this design has the merit that a relatively large workspace/limb-stroke ratio can be achieved thanks to the decomposition of the motions of the output link into the 2-DOF rotation and 1-DOF translation. As with the Tricept, the robot is well suited for use as a plug-and-play module to configure different machines. The dimensional synthesis of the 2-DOF spherical parallel mechanism is carried out by the monotonical analysis of the design variables versus a global conditioning index represented by the mean of the minimum singular value of the Jacobian, leading to the solution of two nonlinear equations due to the limb length constraint and nearly axial symmetry requirement of the kinematic performance. The results of the dimensional synthesis are given via examples.

Kinematic Design of a 5-DOF Hybrid Robot with Large Workspace/Limb–Stroke Ratio

Abstract This paper deals with the conceptual and kinematic designs of a 5-degree of freedom (DOF) reconfigurable hybrid robot. The robot is composed of a 2-DOF parallel spherical mechanism that is serially connected with a 3-DOF open loop kinematic chain via a prismatic joint. Somewhat similar to the well-known Tricept robot, this design has the merit that a relatively large workspace/limb–stroke ratio can be achieved thanks to the decomposition of the motions of the output link into the 2-DOF rotation and 1-DOF translation. As with the Tricept, the robot is well suited for use as a plug-and-play module to configure different machines. The dimensional synthesis of the 2-DOF spherical parallel mechanism is carried out by the monotonical analysis of the design variables versus a global conditioning index represented by the mean of the minimum singular value of the Jacobian, leading to the solution of two nonlinear equations due to the limb length constraint and nearly axial symmetry requirement of the kinematic performance. The results of the dimensional synthesis are given via examples.

ERROR MODELING AND SENSITIVITY ANALYSIS OF RECONFIGURABLE HYBRID ROBOT MODULE TRIVARIAN

For the sub-6 DOF parallel kinematic machines (PKM),it's necessary to separate the geometric error sources which influence the compensatable and uncompensatable errors of the mobile platform by error modeling for the purpose of supervising the accuracy design and kinematics calibration.By taking the TriVariant,a 5-DOF hybrid robot,as an example,a novel approach for error modeling is presented for such robot. The geometric error sources affecting the uncompensatable position and orientation errors of the mobile platform can be identified through this approach.It can be concluded that the uncompensatable errors of the mobile platform are primarily dominated by the errors of the proper-constraint limb included by both the manufacturing and assembling errors of U joint and the twist errors of the tracks.The effects of the geometric error sources on the uncompensatable errors are also discussed by means of the sensitivity analysis.The analysis results show that the error brought by the deviation of the intersection of the two axes of the U joint takes the greatest effects on the uncompen- satable errors of the mobile platform.

Visual servoing by partitioning degrees of freedom

There are many design factors and choices when mounting a vision system for robot control. Such factors may include the kinematic and dynamic characteristics in the robot's degrees of freedom (DOF), which determine what velocities and fields-of-view a camera can achieve. Another factor is that additional motion components (such as pan-tilt units) are often mounted on a robot and introduce synchronization problems. When a task does not require visually servoing every robot DOF, the designer must choose which ones to servo. Questions then arise as to what roles, if any, do the remaining DOF play in the task. Without an analytical framework, the designer resorts to intuition and try-and-see implementations. This paper presents a frequency-based framework that identifies the parameters that factor into tracking. This framework gives design insight which was then used to synthesize a control law that exploits the kinematic and dynamic attributes of each DOF. The resulting multi-input multi-output control law, which we call partitioning, defines an underlying joint coupling to servo camera motions. The net effect is that by employing both visual and kinematic feedback loops, a robot can quickly position and orient a camera in a large assembly workcell. Real-time experiments tracking people and robot hands are presented using a 5-DOF hybrid (3-DOF Cartesian gantry plus 2-DOF pan-tilt unit) robot.